Field of the Invention
The invention relates to the detection of high contrast images of objects, and in particular to the utilization of an optical system that employs a single CCD camera to differentiate between desired crops and weeds and other noxious plants for the purpose of controlling the spray of herbicide and fertilizer.
A major change is now occurring in the agricultural community. This change is called "precision farming," "computer-aided farming" or "site-specific crop management." Instead of using an average value for the entire field and applying chemicals accordingly, vehicle management and location, a real-time analysis of soil chemical composition, computer control of product application and crop yield measurements to the resolution of few meters, are all being investigated to improve yields, lower costs and to lessen environmental concerns.
The difficulty of utilizing infrared (IR) imaging devices for the detection of cold targets is recognized in U.S. Pat. No. 5,132,802 which issued Jul. 21, 1992 to Osthues et al entitled "High Contrast Image Apparatus Employing Optical Filters to Cause Each Image Pick-Up Element to have its Maximum Sensitivity in a Different Spectral Range." The apparatus of the Osthues et al patent is described in the embodiment which is directed to the detection of military objects which may be surrounded by natural vegetation and may be camouflaged.
The Osthues et al system utilizes two image pick-up means which are selected to have two different wavelength ranges. One of the wavelength ranges is selected so that the reflection characteristics of the object and of the surrounding natural vegetation are as similar as possible. In this patent the reflection from the vegetation is considered to be clutter. The second wavelength is selected so that there is the greatest possible difference between the reflection behavior or the object to be detected and that of the vegetation.
Two separate CCD (Charge-Coupled Device) cameras are utilized so that each camera receives and operates on one of the two wavelengths. The first wavelength, which is selected so that the characteristics of the object and the natural vegetation are similar as possible, lies in the near infrared (NIR) range. This wavelength lies between 0.7 .mu.m and 2 .mu.m. The second wavelength, which is utilized to distinguish the background of the object, lies between 0.2 .mu.m and 0.6 .mu.m.
The Osthues et al patent shows two separate embodiments for implementation of that optical device. One of the implementations shows the use of two separate lens and filter combinations, each one of which is used to convey its image to a separate CCD camera. The second embodiment utilizes a single lens and two CCD cameras which are displaced so that one of the camera devices is positioned horizontally and the other is positioned vertically or at right angles. The light from the lens is then directed onto an optical beam-splitter. The beam-splitter splits the beam and directs a portion of it onto a filter providing the first wavelength to one of the cameras and a different filter for providing the second wavelength to the other camera.
It is well known that the ratio of the reflection of light from the near-infrared (NIR) and the red (R) portions of the spectrum is a reliable detector of living plants against various backgrounds of soil and crop residue. For example, in the article "Visible and Near-Infrared Plant, Soil, and Crop Residue Reflectivity for Weed Sensor Design" by B. Nitsch et. al. in the ASAE Proceedings of the 1991 International Summer Meeting at Albuquerque, N.M., on Jun. 23-26, 1991, Paper No. 913006, the ratio of NIR/R is called the "vegetative index." Another alternative ratio that is useful is (NIR-R)/(NIR+R) which is termed the normalized difference ratio is described in this same article. The ratio of NIR/R may be utilized where the NIR is obtained from a portion of the spectrum from approximately 700 nm to 800 nm, and the red is obtained from a region around 600 nm. This may be used to differentiate plant growth from background soil and crop residue. Bandpass filters may be placed in front of photo-detectors to obtain the signals from the various spectral regions. This works well when the detection of living plants is the only requirement. However, when weed species must also be detected, identified and differentiated from the living crops, imaging techniques must be used. Two black and white CCD cameras with two separate filters could be used, but this can be a costly solution. In addition to the additional cost, this creates an alignment problem, especially on close objects since parallax errors will change the image alignment as a function of the distance of the object.
Another paper in the Transactions of the ASAE (American Society of Agricultural Engineers) is entitled "The Use of Local Spectral Properties of Leaves as an Aid for Identifying Weed Seedlings and Digital Images" by E. Franz, et al. It appeared in Volume 34(2): March-April 1991. This paper also discusses the use of NIR reflectance for plant discrimination and the use of a CCD camera and photographic filters to extract spectral data from vegetative and non-vegetative regions.
In order to be commercially feasible for agricultural applications, the utilization of expensive cameras and optical systems must be minimized. In addition, a system which utilizes more than one CCD camera tends to be bulky and somewhat unmanageable.